1. Field of the Invention
The present invention relates to an image forming apparatus employing a roller transfer method such as an electrophotographic copying machine, a laser printer and a facsimile apparatus, and more particularly, to an image forming apparatus where the contact between a transfer unit and a photoreceptor drum can easily be canceled.
2. Description of the Prior Art
In an image forming apparatus employing an electrophotographic method, a latent image formed on the surface of a photoreceptor drum is developed by charged toner to form a toner image, and the toner image is transferred onto a sheet of paper to obtain a toner image on the sheet. As a method of transferring a toner image from the photoreceptor drum onto the sheet, a roller transfer method is known. According to the roller transfer method, a transfer roller is arranged opposite to the photoreceptor drum, and to transfer the toner image, a voltage of a polarity reverse to that of the toner image carried by the photoreceptor drum on its surface is applied to the transfer roller when the sheet passes between the photoreceptor drum and the transfer roller. The roller transfer method is divided into a contact type and a non-contact type according to whether or not the transfer roller is in contact with the surface of the photoreceptor drum.
As a method of holding the transfer roller so as to be out of contact with the photoreceptor drum in a transfer unit employing the non-contact type roller transfer method, for example, a gap maintaining roller as shown in FIG. 1 is used. To each end of a roller shaft 52 to which a transfer roller 51 is attached, a gap maintaining roller 53 having a diameter greater than that of the transfer roller 51 is attached, and the periphery of the gap maintaining roller 53 is rotatively in contact with the surface of the photoreceptor drum 54 so that a predetermined gap C is left between the transfer roller 51 and the photoreceptor drum 54.
Referring to FIG. 2, there is shown a schematic front view of a transfer unit using the gap maintaining roller 53. Below the photoreceptor drum 54, the transfer roller 51 is arranged so as to be out of contact with the drum surface by the above-described method. In the upstream side of the transfer roller 51, a pre-transfer guide 55 having a guide plate 55a for guiding a sheet P to the surface of the photoreceptor drum 54 is arranged. Above the guide plate 55a, a pre-transfer roller 56 for sending the sheet P to the photoreceptor drum 54 at a speed equal to the movement speed of the surface of the photoreceptor drum 54 is arranged so as to be substantially in contact with the upper surface of the guide plate 55a. The pre-transfer roller 56 is supported independently of the photoreceptor drum 54 and the transfer roller 51. In the upstream side of the pre-transfer guide 55, a pair of paper feeding rollers 57 are provided for guiding a sheet fed by a non-illustrated paper feeder to the upper surface of the guide plate 55a.
The photoreceptor drum 54 rotates by a driving force of a non-illustrated motor. It is necessary for the transfer roller 51 and the pre-transfer roller 56 to rotate at a predetermined speed in synchronism with the rotation of the photoreceptor drum 54. For this reason, a drum gear 58, a transfer roller gear 59 and a pre-transfer roller gear 60 are attached as shown in FIG. 3 to the rear side end portions of the photoreceptor drum shaft, the transfer roller shaft and the pre-transfer roller shaft, respectively. The drum gear 58 and the transfer roller gear 59 directly engage with each other, and the transfer roller gear 59 and the pre-transfer roller gear 60 engage with each other through two idle gears 61 and 62. With this arrangement, the rotation of the photoreceptor drum 54 is transmitted to the transfer roller shaft and to the pre-transfer roller shaft, so that the photoreceptor drum 54, the transfer roller 51 and the pre-transfer roller 56 rotate in the directions of arrows a, b and c, respectively, as shown in FIG. 2.
In an image forming apparatus such as a copying machine, it is necessary to detach the photoreceptor drum and a unit such as a developer unit arranged around the photoreceptor drum at a maintenance work. In a conventional apparatus, these units are detached and attached from the front side of the apparatus by opening the front cover. Specifically, to detach and attach the photoreceptor drum, the photoreceptor drum is moved forward and backward as shown by the arrow d in FIG. 1. In an apparatus having a transfer unit of the non-contact type roller transfer method using the gap maintaining roller, the drum surface may be damaged when the photoreceptor drum is detached or attached since the gap maintaining roller is always in contact with the surface of the photoreceptor drum.
Moreover, when a paper jam occurs during copying, the sheet jamming inside the apparatus should be removed. The removal of the jamming sheet is also made from the front side of the apparatus by opening the front cover. However, the sheet cannot easily be removed because of the small gap between the transfer roller and the photoreceptor drum surface and the presence of the gap maintaining roller which is in contact with the drum surface.
These disadvantages occur not only in a transfer unit of the non-contact type roller transfer method but also in a transfer unit of the contact-type roller transfer method. In particular, when paper jam occurs during copying, it is very difficult to remove the sheet which is nipped between the transfer roller and the photoreceptor drum.
To facilitate the above-mentioned works, a machine of a clamshell structure is used which has an openable and closable upper portion. In a machine of the clamshell structure, since the upper portion of the machine is separated from the lower portion at the sheet passage, the removal of a sheet jamming at the sheet passage and the attachment and detachment of the photoreceptor drum are facilitated, and the drum surface is less likely damaged.
However, the opening and closing of the clamshell structure is onerous to users, and it diminishes the efficiency to open the clamshell structure every time a paper jam occurs. Therefore, it is desirable to provide a transfer unit where the gap maintaining roller of the non-contact type roller transfer method or the transfer roller of the contact-type roller transfer can be separated from the photoreceptor drum surface when necessary in order that jammed sheets can be removed without any need to open the clamshell structure.
After a jammed sheet is removed in such a manner, the transfer roller gear, the idle gears and the pre-transfer roller gear should appropriately be engaged with one another when the transfer unit is arranged again so that the gap maintaining roller or the transfer roller is brought in contact with the photoreceptor drum. These gears are of small diameter since they are arranged in a small space, and unless their relative positions are appropriately set, the teeth of the gears do not correctly engage. In particular, at the idle gears of small diameter, a slight variation in position would break the correct engagement of the teeth. Consequently, the pre-transfer roller does not rotate at the predetermined speed. If the rotation speed of the pre-transfer roller changes, a difference is caused between the surface speed of the photoreceptor drum and the conveying speed of the sheet, so that the toner on the drum surface is rubbed by the sheet to damage the toner image.